Frequency division multiplexing (FDM) transmission schemes such as Orthogonal Frequency Division Multiplexing (OFDM), single carrier Frequency Division Multiple Access (FDMA) or distributed FDMA such as interleaved FDMA with multiple terminals will become increasingly important e.g. for future evolutions of air interfaces for mobile radio systems. Those radio systems are currently under discussion e.g. in Third Generation Partnership Project (3GPP) Technical Specification Group (TSG) Radio Access Network (RAN), for Wireless Local Area Networks (WLANs) e.g. according to standard IEEE 802.11a, or for a 4th generation air interface.
Given the licensed bandwidth, transmission capacity from network providers e.g. for picture uploading or video communication has to be as high as possible for all users to serve as many subscribers as possible. Further the quality of service experienced by the user and especially the coverage of the service is an important property demanded by the user. So an access scheme shall work well at the cell borders of networks with frequency reuse.
In cellular systems with a frequency reuse factor of 1 the signal to interference ratio at the cell border can approach the factor 1 or 0 dB, so that no useful transmission from a user terminal to the base station can be kept up if a user terminal from a neighboring cell is near to the considered user terminal and sends with the same power on the same frequencies.
Therefore in CDMA (CDMA=Code Division Multiple Access) a soft handover exists and the user terminals always use a different (terminal specific) scrambling code in the uplink. The reception is then possible using the spreading gain from CDMA. As is known due to the strong interference the uplink capacity is considerably limited.
In FDM transmission, frequency groups are allocated to a user terminal instead of codes in CDMA transmission. In FDMA orthogonal transmission schemes, frequencies are also allocated to a user terminal. So in these schemes in contrast to CDMA transmission, interference can be planned and avoided. For these orthogonal transmission schemes the problem at the cell border has to be solved as well.
A known concept of frequency planning for the cells is giving each whole cell a distinct frequency band.
However, frequency distribution to the different cells reduces the available uplink resources per cell very considerably e.g. by a factor of ⅓ or 1/7 and thus the overall system throughput. It is a waste of resources for the inner area of a cell.
A frequency reuse of e.g. ⅓ only in the outer part of the cell is possible but still wastes too much resources.
A possible concept for coordination of the interference between cells of a network with frequency reuse offering a good usage of the available resources is to subdivide the overall frequency resource into frequency subsets. In every cell a dedicated frequency subset is used with a power restriction. This dedicated frequency subset is assigned by neighbouring cells to user terminals approaching this cell.
Such a concept for the downlink is e.g. disclosed in the document R1-05-0594 with the title “Multi-cell Simulation Results for Interference Co-ordination in new OFDM DL” presented at 3GPP TSG RAN WG1 LTE Ad Hoc on LTE in Sophia Antipolis, France, 20-21 Jun. 2005. For the uplink, such a concept is disclosed in the document R1-05-0593 with the title “Interference coordination for evolved UTRA uplink access” presented at RAN1 AdHoc on LTE, Sophia Antipolis, France, 20-21 Jun. 2005.
Independent of the distribution of user terminals within a cell, the user terminals have to be scheduled to subcarriers in a way to fully exploit the advantages of interference coordination in a multi-cell scenario, i.e. to guarantee a high cell throughput, and at the same time to guarantee a minimum bitrate performance for the individual user terminals.
The object of the invention is thus to propose a method for scheduling of user terminals to subcarriers in a multi-cell or multi-sector network using FDM transmission that guarantees a high cell throughput and a minimum bitrate performance for the individual user terminals.
This object is achieved by a method according to the teaching of claim 1, a base station according to the teaching of claim 5, a user terminal according to the teaching of claim 6 and a network according to the teaching of claim 8.
The main idea of the invention is to measure or to model by means of the signal to interference ratio measured by user terminals the data throughput of said user terminals dependent on the frequency subset and to allocate subcarriers preferably to user terminals with a high signal to interference ratio on said subcarriers.
Furthermore, subcarriers are allocated in clusters to the user terminals, whereby all clusters offer the same data throughput for the respective user terminal.
Further developments of the invention can be gathered from the dependent claims and the following description.